Physicians make use of catheters today in medical procedures to gain access into interior regions of the body to ablate tissue areas. It is important for the physician to be able to accurately steer the catheter to the ablation site. Once at the site, it is important for the physician to control the emission of energy within the body used to ablate the tissue.
The need for accurate steering and precise control over the catheter is especially critical during procedures that ablate tissue within the heart. These procedures, called electrophysiology therapy, are becoming increasingly more widespread for treating cardiac rhythm disturbances, called arrhythmias.
During these procedures, a physician steers a catheter through a main vein or artery (which is typically the femoral artery) into the interior region of the heart that is to be treated. The physician then further manipulates a steering mechanism to place the electrode carried on the distal tip of the catheter into direct contact with the tissue that is to be ablated. The physician directs radio frequency (RF) energy from the electrode tip through the tissue to an indifferent electrode to ablate the tissue and form a lesion.
Some clinicians have suggested the use of microwave energy for cardiac ablation. For example, Langberg U.S. Pat. No. 4,945,915 proposes the use of a helical microwave antenna fed by a coaxial line to thermally ablate cardiac tissue. The radiation heating patterns that microwave energy propagate can, in theory at least, form lesions that are deeper than the lesions formed by the conductive heating patterns generated by conventional RF energy.
The ability of microwave energy to form deeper lesions also raises challenges in antenna system design. To gain all the benefits of using microwave energy, the clinician must be able to control the type and the distribution of heating patterns propagated at the intended lesion site. If a microwave antenna system also propagates unintended conductive heating patterns, the temperature of the ablation site can be quickly elevated above a critical isotherm (thought to be in the range of 53 to 55 degrees C.) before the desired lesion depth is achieved. Irreversible tissue damage can result. The same unintended conductive heating patterns can also quickly heat the blood pool around the ablation site, causing undesired coagulation.
Ablation systems and processes using microwave energy will not find widespread clinical use, if they cannot be made and controlled to minimize the propagation of conductive heating patterns. They will also fail to find widespread use, if the microwave antenna cannot be conveniently steered and positioned to the desired ablation site within the heart.